This invention relates to a teletext decoder having a microcomputer for creating a pixel map, and having a register array for translating a pixel word of the pixel map to a color code and another code called a transparency code.
Teletext is a general term for a television-based communication technique. A horizontal line may be utilized for broadcasting textual and graphical information encoded in a digital binary representation. Teletext may be sent during the vertical blanking interval, when no other picture information is sent. The teletext binary information includes control and display digital information serially organized in data blocks. The organization of the binary information in the broadcasted signal is determined by the standard employed by the broadcaster. By way of an example only, references are made here to the proposed NABTS (North American Broadcast Teletext Specification).
In the NABTS each horizontal line containing teletext data is referred to as a packet. The binary data is divided into bytes; each byte includes eight binary units (bits). The first eight bytes of each packet are collectively known as the packet header. Three bytes of the packet header define the channel number and each channel is organized into pages. Each page is made up of a number of packets.
After its reception by the television receiver, the digital data included in the video signal is processed by the teletext decoder. Then the digital data is extracted from the video signal by a data slicer providing a stream of bits to a data processor (sometimes referred to as the prefix processor). The data processor may be made to receive user-initiated commands specifying the desired information for display. The data processor buffers in a memory the data contained in the teletext channel selected for displaying. The buffered data is processed and provided to a display processor which outputs the displaying signals. When a television picture tube (CRT) is used as an image displaying device, the display processor has to output the displaying signals periodically for maintaining the image on the television screen.
One feature of the invention is a microcomputer which processes the buffered teletext data words obtained from the data processor to create a pixel word map for usage by the display processor. The pixel word map includes pixel data words. Each pixel word defines the display information of one picture element called pixel. The pixel word may select a display word which characterizes a pixel. The possible number of display words which may be selected by a pixel word may equal to the binary value of the pixel word. The display processor reads the pixel data words from the pixel word map sequentially and provides the display with signals characterizing the pixels. This arrangement in which the display processor is provided with the pixel word map obviates the usage of a character generator and makes the design of the display simple.
Another feature of the invention is the bit arrangement of the display word selected by the pixel word. The display work of the invention includes a color code and a separate transparency code. The transparency code may be used to control the source of picture information provided to the red, green and blue drive circuits of the cathode ray tube. When the transparency code has a given value, the color code associated with the display word is the source of picture information. When the transparency code has a second value, a source other than that of the teletext signal provides the pixel picture information. Such other source may be one that sends a picture video signal at times when the teletext video signal is not sent.
One advantage of utilizing a transparency code which coexists with the color code is that the circuit utilized for translating the streamlined pixel word to a display word is relatively simple. Such circuit may be constructed from an array of registers addressed by a pixel word. A register selected by a pixel word may include a color code and a coexistent transparency code.
Another advantage of utilizing a display word that contains a transparency code which coexists with the color code in the same display word is that it may be possible to permit the user to override the transparency code. In such a case, the color code of the display word determines the characteristics of the pixel.